This application claims the priority of Patent Application No. 198 13 604.8 filed in Germany on Mar. 27, 1998, the subject matter of which application is incorporated herein by reference.
The present invention relates to an arrangement for the precisely measuring distance, in particular, for measuring the level of a liquid in a tank.
There exist various arrangements for measuring the filling level of a liquid in a tank, in particular in large tank plants, some involving various mechanical arrangements. Among the existing arrangements are arrangements incorporating a radar device which measures, starting from a fixed height, the distance to the liquid surface, i.e., the radar device measures the length of the empty space between the fixed height and the top of the liquid level in the tank. The radar device may be a FMCW radar device having operating frequencies in the microwave range.
The operating frequency of the radar device is tuned continuously, for example in equidistant frequency steps which ideally form over time a stair-like course with small steps along a linear ramp of the operating frequency. The operating frequency typically is generated in a frequency-variable, controllable oscillator, in particular a VCO or is derived from the output signal of such an oscillator through frequency multiplication.
The oscillator is normally actuated by a digital/analog converter by gradual change of the control voltage. In order to control the actual oscillator frequency, the oscillator output signal can be supplied to a counter by using the constant frequency of a fixed frequency oscillator as frequency standard, which can, if necessary, be followed by an initial frequency division. If the measured oscillator frequency deviates from the listed frequency, the tuning voltage for the digital/analog converter can be adjusted by a correction value. The correction values can also be determined during a calibration phase and can be considered in the oscillator control in the form of corrected tuning voltages of a non-linear oscillator characteristic.
Calibrating the oscillator characteristic by determining corrected tuning voltages at several or all of the frequency stages of the oscillator frequency tuning range requires a time interval which, due to the frequency measuring time of the counter, as a rule far exceeds the duration of a tuning operation to obtain a sufficiently high measuring accuracy of the counter in the frequency range for the filling level. Due to the fact that the operating conditions of the oscillator will change over time, a new calibration has to be carried out at short time intervals.
It has, moreover, been found that after specifying a new control voltage value, the oscillator does not change to a new, constant frequency value, but that following a quick jump in the frequency, the oscillator frequency continues to change in the following interval with constant control voltage. These undesirable frequency changes are primarily caused by the temperature dependence of the oscillator characteristic. In this case, temperature fluctuations are caused not only by changes in the environmental temperature, but also, and above all, by changes in the power consumption of the oscillator itself or in neighboring control circuits. The effects of these temperature changes on a semiconductor substrate or a carrier ceramic have much shorter time constants than the interruptions caused by changes in the environmental temperature.
In view of the above, the mean value of the measured frequency is not identical to the actual, drifting oscillator frequency. The calibration of the oscillator characteristic is less secure than is permissible for the desired measuring accuracy with a range resolution in the order of magnitude of one millimeter or a few millimeters. The same problems occur if a frequency ramp with continuous linear rise over time is to be generated in place of the step-by-step frequency tuning.
It is, therefore, the object of the invention to specify an arrangement for the distance measuring, in particular the filling level measuring, by using a FMCW radar device, which arrangement ensures an improved, targeted adjustment of the changeable operating frequency and thus a reliable and precise distance measuring.
With the object in view, the present invention resides in an arrangement for carrying out a precise distance measurement, in particular for measuring the tilling level of a liquid inside a tank by means of a FMCW radar device, in which a frequency-variable oscillator is actuated with digital means to generate a transmitting frequency which can be tuned over a predetermined frequency range. The digital means comprise a digital frequency generator which derives in predetermined frequency steps a reference signal from a fixed-frequency oscillator signal and the frequency of the frequency-variable oscillator is adjusted in a phase-locked loop by linking it to the reference signal.
The arrangement according to the present invention makes it possible to achieve, at a low cost, a reliably high measuring accuracy in the range of one to several millimeters for measuring distances up to approximately 50 m, this being done by generating a linear frequency ramp with predetermined slope or exact frequency steps, which are for the most part independent of external influences such a temperature and component scattering, as well as the less than ideal characteristics of the oscillator. In this case, the transmitting frequency range advantageously can be above 1010 Hz. The invention can, for the most part, use standard electronic components, particularly components from the digital electronic field and the high-frequency electronic field, such as DDS modules and/or PLL modules. Components requiring a discrete design and/or implementation are used for special application cases. Available components can be modified.
The method of generating the FMCW radar frequency ramp, continuously or in steps, which is used in accordance with the invention makes advantageous use of the principles of the digital direct synthesis (DDS), known per se, the so-called fractional-N-frequency generators and/or the phase-locked loops (PLL). As a rule, inexpensive and readily available prefabricated components can be used for this. A detailed explanation of the aforementioned methods can be found, for example, in the literature references [1] and [2], set forth at the end of this specification.
The noise portion of the transmitting signal, which is based on phase fluctuations of the oscillator output signal, is reduced at the same time, inasmuch as the phase-locked loop adjusts the oscillator output signal to a constant phase position with respect to the reference signal that is present with high precision.
The phase-locked loop typically has a divider with a whole number divider ratio N, so that the oscillator signal is adjusted to N-times the reference signal. The accuracy of the oscillator frequency essentially depends only on the accuracy of the reference frequency. The non-linear connection between tuning voltage and oscillator frequency, as well as the temperature dependence of the oscillator, do not play a role in this case. A phase-locked loop makes it possible to adjust the transmitting frequency quickly to a specific value, determined by the reference frequency and the dividing factor, and to stabilize it against changes caused by temperature fluctuations. If, for a constant dividing factor of the phase-locked loop, the reference frequency is changed linearly or in defined, predetermined steps over a period of time, then a special linear frequency ramp or a defined sequence of frequency steps with exact step frequencies also results for the oscillator signal and the transmitting signal, derived thereof either directly or preferably after the frequency division.
The use of a phase-locked loop causes as shift in the linearity requirements from direct control by the oscillator to generating an exact reference signal, which can be located in a much lower frequency range. The frequencies for the transmitting signal and thus also the oscillator signal should be adjustable in very small steps, so that the derivation of the reference signal through frequency division of a frequency standard cannot be realized with acceptable expenditure in the frequency range considered herein. However, if a high divider factor N in the phase-locked loop and a low reference frequency in the range of several MHz are advantageously selected, then the reference signal advantageously can be generated digitally and with high accuracy by using the above described techniques.